Problem: What is the least common multiple of 30 and 21? Another way to say this is: $\lcm(30, 21) = {?}$
Explanation: The least common multiple is the smallest number that is a multiple of 30 and 21. We know that 30 x 21 (or 630) is a common multiple, but is it the least common multiple? Write out the multiples of 30 until we find a number divisible by 21. 30, 60, 90, 120, 150, 180, 210, So, 210 is the least common multiple of 30 and 21.